1. Field of the Invention
The present invention relates to a control system and methods for controlling a multiple operation production process having time constraints, and particularly to a control system and methods for controlling a production process with overlapping time constraints.
2. Description of the Related Art
Semiconductor manufacturing processes are often restricted by stringent production process requirements. Time constraints are one type of production process requirement that should be considered during the production of a lot of semiconductor devices. If time constraints are breached, for example, when the queue time of a lot exceeds process time constraints, the lot may need to be reworked or even scrapped. FIG. 1 illustrates a production line 100 with a plurality of operations (O1, O2, O3, . . . , On−2, On−1, On) , each of which includes at least one process step.
Generally, there are four types of time constraints in production processes: dual-operation, multi-operation, continuous, and overlapping time constraints. The dual-operation time constraint is a basic type of time constraint between two individual operations. FIG. 2A is a schematic diagram of a dual-operation time constraint. In FIG. 2A, production timeline 210 has two operations O1 and O2 with time constraint TC1 therebetween, that is, operations O1 and O2 must be performed on a product before the expiration of a period of time identified by time constraint TC1. As shown in FIG. 2B, the multi-operation time constraint is an extension of the dual-operation time constraint. In FIG. 2B, the production timeline 220 has four operations O3, O4, O5 and O6, with time constraint TC2 therebetween, that is, operations O3, O4, O5 and O6 must be performed on a product before the expiration of a period of time identified by time constraint TC2.
FIG. 2C is a schematic diagram of a continuous time constraint. The continuous time constraint is a combination of time constraints corresponding to different operation sets. In FIG. 2C, the production timeline 230 has three operations O7, O8 and O9, with time constraint TC3 existing between operations O7 and O8, and with time constraint TC4 existing between operations O8 and O9. FIG. 2D is a schematic diagram of an overlapping time constraint. The production timeline 240 has four operations O10, O11, O12 and O13. An overlapping time constraint may comprise any combination depending on process need, and may include dual-operation, multi-operation and continuous time constraints. In FIG. 2D, time constraint TC5 exists between operations O10 and O11; time constraint TC6 exists between operations O11 and O12, and time constraint TC7 exists between operations O12 and O13. In addition, time constraint TC8 exists between operations O10 and O12 and time constraint TC9 exists between operations O11 and O13. In this case, time constraint TC8 overlaps time constraints TC5 and TC6, time constraint TC9 overlaps time constraints TC6 and TC7, and time constraints TC8 and TC9 overlap each other.
Verifying the timeliness of the completion of process operations, by comparing the various completion times with a corresponding time constraint, addresses timeliness concerns associated with the identified process operations. However, comparing individual operational milestones (e.g., operation completion times) against a time constraint ignores many of the complexities of managing the manufacture of multiple lots through a production process. For example, when time constraints overlap, the difficulty of managing the production process across the various lots in process increases dramatically. With the lack of an effective control mechanism, lots may be accepted to the production line simply to satisfy some time constraints. Under these uncontrolled conditions, the successful completion of one or more lots may be threatened if the lots in process encounter a bottleneck and a remnant time-constraint window is insufficient to cover the required process time to complete the manufacture of each respective lot.
Currently, process managers make separate lot dispatch decisions such that the lots avoid overlapping time constraints. This process management approach simplifies the difficulty associated with dispatch decisions. However, this process management approach ignores the dynamic nature of a manufacturing process. Specifically, this approach ignores production variance between the relative operation completion times between production lots. Thus, the inevitable production variance between production lots adds to the difficulty of achieving an optimized distribution of Work-In-Process (i.e., the lots) over the entire production schedule. Consequently, focusing on dispatch decisions alone inevitably impacts the flexibility of the original production schedule, causing higher product variability across the various completed lots.